(CNN) -- Consumer advocate, lawyer, presidential candidate and critic of Democrats and Republicans alike, Ralph Nader bears many titles. Yet as he mounts his fourth bid for the presidency, Nader's candidacy is shadowed by the 2000 election and whether he helped elect President George W. Bush.

As the Green Party candidate in 2000, Nader won 2.74 percent of the national vote, placing third. But many Democrats blame him for siphoning off votes in key states, especially Florida, that might have gone to Democratic nominee Al Gore. Bush won Florida by 537 votes and won the election after a 5-4 decision in the U.S. Supreme Court settled a protracted dispute over the Florida vote.

The possibility of Nader's candidacy contributing to Bush's re-election fuels a ferocious debate among Democrats, Nader supporters and other liberal activists.

Some former Nader supporters urged him to skip the 2004 election and are now saying a vote for Nader is a vote to re-elect Bush. Other liberal activists paint Nader as an out-of-touch liberal warrior driven by ego.

Nader disagrees. The Supreme Court, he says, cost Gore the election and he blames the former vice president for running a lackluster campaign.

"The Democrats should stop whining and go to work," he said in a CNN interview on March 30. "They should be landsliding Bush" in 2004.

Bush, he said, has a clear record against workers, consumers and the environment, and is a "big corporation in the White House disguised as a human being."

He contends both parties are captive to corporate interests. When announcing his campaign February 22 on NBC's "Meet The Press," he said he is running to "challenge the two-party duopoly" he believes is damaging American democracy. He also criticized the treatment of third party and independent candidates as "second-class citizens."

In Nader's view, the Democratic Party is part of the problem, having lost touch with its progressive heritage.

"The liberal intelligentsia has allowed its party to become a captive of corporate interests," he said.

A lifetime of advocacy

Nader was born in Winsted, Connecticut in 1934, one of Rose and Nadra Nader's four children. His father, a Lebanese immigrant, owned and operated a restaurant where Nader worked in his youth. He has spoken Arabic since he was a child.

He attended Princeton University and graduated magna cum laude with a bachelor of arts degree in 1955. He went on to law school at Harvard University, graduating in 1958.

After a six-month stint in the Army in 1959, Nader traveled through Latin America, Africa and Europe. Nader began practicing law in Hartford, Connecticut. His career as a public advocate started with an article in The Nation titled "The Safe Car You Can't Buy." He also lectured on history and government at the University of Hartford from 1961-1963.

He was hired in 1964 by then-Assistant Secretary of Labor Daniel Patrick Moynihan as a paid consultant to the Labor Department. He worked on a federal government study on auto safety and helped a Senate subcommittee on the same subject.

In May 1965, Nader left the Labor Department to work on a book about auto safety. "Unsafe at Any Speed" launched him into the public spotlight. The book, published in November 1965, documented safety defects in U.S. cars and criticized the automobile industry's safety practices, specifically targeting the Corvair, which was built by General Motors.

Helped by the revelation that General Motors hired a private detective to investigate Nader's private life, the book became a best seller. Nader subsequently sued GM for invasion of privacy and eventually received $425,000 in an out-of-court settlement.

He used the money to expand his advocacy efforts. Nader's research on auto safety and his lobbying in Washington helped push Congress to pass the 1966 National Traffic and Motor Vehicle Safety Act. He also lobbied for the 1967 Wholesome Meat Act, which called for federal inspections of beef and poultry and imposed standards on slaughterhouses, the 1967 Freedom of Information Act and the 1970 Clean Air Act.

In 1969, he helped found the Center for Study of Responsive Law (CSRL), a nonprofit organization staffed mostly by college, graduate and law students. Those students became known as "Nader's Raiders" and studied and issued reports on a variety of consumer issues.

The CSRL is now a consortium of a number of nonprofit groups, including the Consumer Project on Technology, Freedom of Information Clearinghouse, and the League of Fans, which examines how sports businesses operate.

In 1971, Nader founded Public Citizen, a nonprofit national consumer advocacy organization that aims to represent consumer interests in Congress, the executive branch and the courts. Public Citizen remains in operation and is now headed by Joan Claybrook, a Nader protégé who headed the National Highway Traffic Safety Administration from 1977 to 1981.

Nader resigned as director of Public Citizen in November 1980 and began to focus on issues relating to trade and corporate power. That same year, he founded the Multinational Monitor, a magazine that examines multinational corporations and their relationship with developing nations, labor and the environment.

The 1980s were a difficult time for Nader. His efforts to convince the federal government to regulate issues in the public interest conflicted with the Reagan administration's efforts to limit the power of federal agencies. Nader's older brother Shafeek died of prostate cancer in 1986. After his death, Nader developed a case of Bell's palsy that paralyzed the left side of his face for several months.

Nader dealt with the paralysis with his deadpan sense of humor, telling audiences at public events that "at least my opponents can't say I'm talking out of both sides of my mouth."

He has written or edited more than 25 books since the publication of "Unsafe At Any Speed." His personal financial disclosure in 2000 listed his net worth at close to $4 million, including $1 million of stock in tech giant Cisco Systems.

Such revelations prompted critics to label Nader a hypocrite for railing against corporations while personally profiting from its success. But in his disclosure, Nader said that aside from "modest personal expenses," he uses his income as a "de facto philanthropic fund" for his projects. He also said that he has consistently donated 50 percent of his adjusted gross income to charitable institutions, amounting to several million dollars since 1967.

"In short, monies I earn are for strengthening civil society," he said.

Nader is well known for his modest lifestyle. He lives alone in a Washington apartment and does not own a car. He never married and has no children. He told The New York Times in 1995 that he didn't want to be an absentee father or husband. "That would have been terrible," he said.

Activist turned candidate

Always registered as an independent, Nader first entered the political arena as a candidate in 1992 as a write-in candidate for president. In 1996, he agreed to run as the Green Party's candidate for president. He hardly campaigned or raised money, and received 685,128 votes, or 0.71 percent of the national popular vote.

Nader was the Green Party nominee again in 2000. He actively campaigned and raised money, and was supported by several high-profile celebrities. He significantly increased his national vote total, receiving 2,882,955 votes, or 2.74 percent. But it was the 97,488 votes Nader received in Florida that brought him the wrath of Democrats once Gore lost the state and the election.

Nader's campaign Web site lists multiple explanations for Gore's loss, including Bush's recount strategy in Florida, the controversial butterfly ballot in Palm Beach County, the purging of tens of thousands of non-ex-felons from the Florida voter rolls, and Gore's campaign.

"Gore beat Gore," Nader said in a December 2003 CNN interview. "He didn't get Tennessee, his home state. That would have made him president. And he blundered in Florida and didn't ask for a statewide recount."

Nader says his 2004 candidacy will act as a "second front" against Bush and attract Republican voters angry with Bush over issues like corporate responsibility and the rising federal deficit.

"There's a real revolt brewing in conservative circles," Nader said in a CNN interview after he announced he was running. "And then there's ... the liberal Republicans, who never liked Bush to begin with."

Levels of support

However, Nader may not have the level of support among liberal activists that he received in 2000. The editors of The Nation published an open letter to Nader in the February 16, 2004, issue urging him not to run.

RalphDontRun.net, a Web site that originally urged him not to run, now urges potential Nader voters to vote for Democratic nominee John Kerry instead.

John Pearce, a Democrat who helped create the Web site, said in a CNN interview that the issues at stake in this election -- Bush's tax cuts, the budget deficit, the potential to nominate a Supreme Court justice -- are too much for progressive voters to risk voting for Nader.

"These are the kinds of issues that unify not just progressives, but centrists, and really are alarming such a wide range of people, that any risk of peeling off a million votes for a Ralph Nader candidacy from a Democratic candidate is something people just aren't willing to even consider this time around," he said.

Even former President Jimmy Carter -- who said Nader was such a trusted friend that he was once allowed to umpire a Carter family softball game down in Georgia -- urged him to get out of the race.

"When I was president, he gave me a lot of advice. And tonight, I want to return the favor by giving him some advice. Ralph, go back to umpiring softball games or examining the rear end of automobiles, and don't risk costing the Democrats the White House this year as you did four years ago," Carter said at a Democratic National Committee fund-raiser in March.

Nader dismisses these arguments as well as other criticism of him as an ego-driven candidate.

"That means they're out of arguments," he said March 30 on CNN. "My compass comes from 45 million Americans who don't have health care after all these years."

Nader says his candidacy is simply another way to advocate issues that he has made his life's work.

"This is a commitment to justice," he said. "I've been working for 40 years on behalf of the health, safety and economic well being of the American people. I don't like citizen groups being shut out by both parties in this city, corporate occupied territory, not having a chance to improve their country."